It is known that steering wheels for motor vehicles can consist of a structure that comprises a metal core buried inside synthetic resin, and an outer covering consisting, for example, of a bivalve shell which forms the part designed to be gripped and the external appearance of the steering wheel.
One of the various procedures employed in the manufacture of steering wheels for motor vehicles uses two shells to form the outer covering, obtained by hot forming of suitable laminar material.
After the edges have been finished by means of mechanical machining, the shells are respectively applied to the opposite sides of the steering wheel consisting of the metal core and the synthetic resin covering.
The two shells making up the covering are then fixed to each other on the steering wheel by means of gluing along their contact line.
The gluing of these shells often proves to be a key step in the production process as a whole, particularly because the thickness of the shells is minimal, generally a few tenths of a millimeter.
Some solutions are known to the background art that would tend to make the gluing of the shells more resistant; one example is described in the patent application EP-A1-1029770 where the edges of the shells forming the covering are mechanically machined in such a way to obtain a male female type complementary joint that couples together when the two shells are applied to the body of the steering wheel.
The solution proposed by the previous patent clearly tends to increase the contact surface along the edge of the shells so as to improve the fixing and the gluing.
The solution presented by the patent application EP-A1-1029770 is inevitably insufficient and difficult to apply when the thickness of the shells is minimal and not easy to machine; in fact the shells often tend to break and to develop cracks during the mechanical machining of the edges to form the coupling surfaces.
A further typical example in which the gluing problem becomes evident is when carbon fibre shells are used.
In the patent application EP-A1-1029771 the fixing phase of the two shells on the body of the steering wheel comprises a previous stage in which the support sections are applied and fixed on the synthetic resin body of the steering wheel.
These sections are shaped to form a base for the support and fixing of the two shells.
In practice the sections are normally made of metal, typically aluminum which permits easy clinching to the body of the steering wheel and a particular aesthetic appearance when the section is left on view as described in the patent application EP-A1-1029771.
On the other hand, however, the use of metal material is particularly disadvantageous in consideration of the great difference in the coefficient of thermal expansion of these materials compared with the materials used to manufacture the coverings.
In fact, as the steering wheels of motor vehicles are subject to high temperature ranges the great difference in the coefficient of thermal expansion of these materials inevitably leads to the two shells forming the outer covering becoming detached.
In addition to the aforesaid drawback, the procedure described in the patent EP-A1-1029771 presents a further disadvantage in consideration of the particular machining that the body of the steering wheel must undergo.
The gluing or clinching of the sections must in fact be carried out with particular precision so as to avoid problems when fitting the shells on the body of the steering wheel and the unsightly appearance of the end product.
Document WO-A2-0005123 describes a steering wheel comprising a decorative covering especially with decorative wood. The steering wheel frame is provided with a foam sheathing in the area (3, 4) which are provided for the decorative covering and which comprise outer dimensions that are smaller than those of the remaining areas. Furthermore, the above document describes that one dimensionally stable decorative covering part is fastened on said foam sheathing and at least one centering means is provided between the foam and the decorative covering part.
The procedure for manfacturing a steering wheel according to document WO-A2-0005123 requires the centering of at least one of the shells on the body of the steering wheel and furthermore the gluing involve not only the shells but also the body of the steering wheel. These are clearly strong drawbacks since the deformations of the material due to the high variance of the temperatures inside the car cause an alteration in the stability of the steering wheel.